Spatial light modulators are arrays of one or more devices that can control or modulate an incident beam of light in a spatial pattern that corresponds to an electrical input to the devices. The incident light beam, typically generated by a laser, can be modulated in intensity, phase, polarization or direction. Some modulation can be accomplished through the use of Micro-Electromechanical System devices or MEMs that use electrical signals to move micromechanical structures to modulate light incident thereon. Spatial light modulators are increasingly being developed for use in various applications, including display systems, optical information processing and data storage, printing, and maskless lithography.
FIG. 1 shows a linear (1-dimensional) array 100 of a number of ribbon-type spatial light modulators (SLM 102) or diffractors. Generally, each SLM 102 consists of a number of active (movable) ribbons are interlaced or paired with a number of static bias ribbons. By displacing the active ribbons by a quarter wavelength (λ/4) relative to the static ribbons light reflected from the active ribbons interferes with that reflected from the static ribbons, and a square-well diffraction grating is formed along the long axis of the array 100. In the embodiment shown, several ribbon pairs are ganged under action of a single channel driver 104 to form a single MEMS pixel 106. By assembling a large number of MEMS pixels 106 and drivers 104, a continuous, programmable diffraction grating results, such as is particularly useful in printing and lithography applications.
A schematic side view of a deflected active ribbon of the SLM 102 of FIG. 1A is shown in FIG. 1B. Referring to FIG. 1B, one shortcoming of existing ribbon-type SLMs 102 is that when a potential difference is applied between an active ribbon 108 and substrate (not shown) the active ribbon is deflected into a parabolic profile as shown. As a result the square-well diffraction grating is established only in a narrow region near the center-line of the array 100 that is truly displaced by a λ/4. Regions outside this optical “sweet-spot” are neither parallel to the surface of the SLM array 100 nor displaced by λ/4 and therefore cannot provide the desired high contrast and high efficiency modulation. For this reason, illumination onto the standard ribbon-type SLM array 100 must be carefully shaped or focused into a line of illumination 110 using, for example, a Powell Lens. A typical rule of thumb is that the width (W) of the illumination 110 should be no more than about a tenth ( 1/10th) of a length (L) of the ribbon 108.
The need to concentrate the illumination along a narrow line in the middle of the array 110 leads to a number of problems. First, as noted above because of the limited “sweet-spot” of the ribbon-type SLMs 102 sufficient contrast is only achieved when the width of the line illumination 110 is an order of magnitude less than the length of the ribbon 108, and the line illumination is precisely located in the middle of the array 100. If the illumination 110 line-width is too large, or misaligned relative to the array 100, contrast as well as the modulation efficiency will be severely reduced. Thus, the complexity and cost of a SLM array 100 using this approach is greatly increased by the need for additional optics, i.e., the Powell Lens, and the need for a mechanism and procedures to precisely align the illumination 110 relative to the array.
A second and more fundamental problem with this approach is that line-illumination concentrates laser power in a thin line having a high power-density, and creating a thermal knife-edge resulting in enormous thermal gradients in the ribbons 108 of the SLM 102. Moreover, as power density is pushed higher in many applications, such as in Computer Thermal Printing (CTP) applications, in an effort to increase throughput, these thermal gradients continue to increase to the point where the ribbons 108 begin to fail. Typically, the failure mode the ribbons 108 is the “Soret effect” in which atoms of a reflective metal, such as aluminum, covering the ribbons physically migrate ‘downhill’ along the thermal gradient from a hotter to a cooler region of the ribbon. This migration of metal atoms can reduce the reflection and hence the efficiency of the SLM array 100, and ultimately shortens useful device life.
Accordingly, there is a need for a new SLM and method of operating the same to provide a programmable diffraction grating without the need for additional, linear illumination optics, and the need for a complex alignment mechanism and/or procedures. It is further desirable that the SLM and method are capable of handling increased illumination (laser) power levels without resulting in extreme thermal gradients that reduce the efficiency and limit the life of the SLM.